Characterization of the thermodynamics and deformation behavior of styrene-butadiene-styrene grafted with polyhedral oligomeric silsesquioxanes

This research study uses a model nanostructure-copolymer system in order to develop a fundamental understanding of how polymers are affected by functionalized nanostructures. For this study, polyhedral oligomeric silsesquioxanes (POSS) was chosen as the model nanostructure and polystyrene-block-polybutadiene-block-polystyrene copolymer (SBS) as the model polymer host. The choice of materials and chemical reaction scheme for grafting the SBS-POSS copolymers allows for reproducible products with the opportunity for wide selection of nanostructure grafting fractions.; In order to examine the effects that the nanostructure's electronic properties have on the host polymer, the organic group of the POSS nanostructures was varied. To facilitate a rigorous comparison, four sterically similar, yet electronically different POSS derivatives were selected (cyclopentyl (Cp), cyclopenyl (Cy), cyclohexenyl (Cye), and phenyl (Ph)). Ph-POSS results in the greatest changes relative to the ungrafted SBS block copolymer because its chemistry has the largest contrast to the block in which it is grafted while simultaneously having the largest affinity toward the ungrafted block.; All four of the cyclic POSS were found to have some affinity toward the polystyrene phase, so iBu-POSS was investigated in order to observe the effects of incorporating a noninteracting nanostructure. Two host morphologies were examined in order to compare noninteracting (iBu) and strongly interacting (Ph) POSS nanostructures. The morphology and phase behavior observed for noninteracting POSS is consistent with simply changing polystyrene content with no noticeable change in chi. Furthermore, local and long-ranged order of the morphology is well-preserved. The interacting nanostructures reduce chi substantially and disrupt the local order of the morphology, which is equivalent to a compatibilizing effect.; Deformation was studied to supplement the previous findings regarding the equilibrium morphology, and give further information on the nature of the interactions between POSS-polymer and POSS-POSS interactions. The mechanical properties for both the grafted copolymers inherently drop because nanostructure grafting reduces the polystyrene content. For the noninteracting iBu-POSS, the reduction in mechanical properties was consistent with the fraction of polystyrene because the nanostructures are noninteracting with the polystyrene block and do not qualitatively change the morphology. This was observed at both low (30°C) and high (70°C) temperatures. The interacting Ph-POSS behavior is more complex. With increasing Ph-POSS, the morphology becomes more disrupted which should result in an additional drop in mechanical properties. However, there is reinforcement at larger concentrations of Ph-POSS. The disrupted morphology of the 10% and 20% Ph-POSS results in a smaller grain size, which together with the presence of POSS crystallites reinforce, or network, the morphology. Low strain reinforcement is more prominent at lower temperatures. At high temperatures (but still below Tg,PS), the drop in mechanical properties from the disrupted morphology order outweighs the low strain reinforcement effects. Some reinforcement is still observed at these higher temperatures due to POSS-POSS effects.